(1) Field of the Invention
The present invention relates to a solid-state imaging device used for a digital camera and the like, and in particular to technology for suppressing image noise in an MOS-type solid-state imaging device.
(2) Description of the Related Art
In general, an MOS-type solid-state imaging device comprises (i) a pixel array that includes a plurality of pixels arranged in rows and columns, and (ii) a signal processing unit that parallelly processes and stores pixel signals of a row of pixels included in the pixel array. During a horizontal blanking period, the signal processing unit parallelly processes pixel signals of a row of pixels and stores these processed pixel signals. Then during a horizontal readout period that follows the horizontal blanking period, these processed pixel signals are read out from the signal processing unit, one by one, in column order of the pixel array. By repeating the above-described set of operations for each row in a frame, pixel signals for one frame of pixels can be read out (e.g., see Japanese Laid-Open Patent Application No. 1-122277).
In recent years, a solid-state imaging device has been used not only for a general digital camera, but also for an in-vehicle camera, a surveillance camera, and various other systems. Consequently, demand is growing for a solid-state imaging device with specifications for functions that have never been available conventionally, such as an ultra-high-speed shutter and an ultra-high-speed readout. While working on the development of a camera under such demand, inventors of the present invention discovered that depending on specifications, noise occurs periodically during every horizontal readout period, possibly causing the following image noise to appear.
FIG. 37 is a diagram for explaining image noise and the cause of the image noise.
FIG. 37 shows an electronic shutter pulse as an example of the cause of the periodic image noise. The electronic shutter pulse is one of drive pulses that are mutually applied, via row-signal lines, to every row of pixels included in the pixel array. Conventionally, the electronic shutter pulse has been applied during the horizontal blanking period (HBLK). In the example of FIG. 37, however, the electronic shutter pulse is applied during a predetermined period within the horizontal readout period, in order to finely adjust an ultra-high-speed shutter and an exposure time. If the electronic shutter pulse is applied during the horizontal readout period as just described, the electronic shutter pulse triggers noise, resulting in the noise entering some of the pixel signals by way of a series of connected circuits. As the noise repeatedly occurs in every horizontal readout period, an image noise 71 appears notably as a vertical line or a vertical belt in a captured image.
It should be noted that the electronic shutter pulse is one example of the cause of the periodic noise. Depending on specifications, pulses other than the electronic shutter pulse may cause such periodic noise.